This invention relates to the field of cardiac resuscitation, and in particular to devices for assisting rescuers in performing chest compression during cardio-pulmonary resuscitation (CPR). Chest compression during CPR is used to mechanically support circulation in subjects with cardiac arrest, by maintaining blood circulation and oxygen delivery until the heart is restarted. The victim's chest is compressed by the rescuer, ideally at a rate and depth of compression in accordance with medical guidelines, e.g., the American Heart Association (AHA) guidelines. One key step for creating blood flow through the heart is to release the chest adequately after each chest compression. The chest should be released sufficiently to create a negative pressure in the chest, to facilitate venous filling of the heart and increased blood flow upon the next chest compression. If the chest is not released adequately, a positive thoracic pressure will remain which will hinder venous return and right atrial filling. Other key CPR parameters are maximal velocity of compression, compression depth, and average velocity. Compression depth and average velocity, together, provide good indication of potential blood flow volume. Maximal velocity of compression is an important factor in proper mitral valve closure and higher blood flow volume.
Sensors have been suggested for detecting the depth of chest compression. An accelerometer (with its output integrated to estimate depth) was disclosed, for example, in Freeman U.S. application Ser. No. 09/794,320, U.S. Pat. No. 6,306,107 and U.S. Pat. No. 6,390,996. Force (pressure) sensors were disclosed, for example, in Groenke U.S. Pat. No. 6,125,299. Force sensors provided no way of determining absolute displacement, as the compliance of the thoracic cage varies considerably from person to person. Accelerometers do not provide an indication of whether or not the chest is being released. They calculate displacement by double integration, which can result in a significant DC offset. U.S. Pat. No. 6,306,107 attempted to address the DC offset problem by incorporating a force sensor as a switch to indicate onset and conclusion of compression. The prior art has also employed mechanical pressure gauges to indicate to the rescuer the amount of force or pressure being applied to the chest. But these prior art uses of an accelerometer and/or force sensor have not provided a good solution to providing the rescuer with useful feedback as to whether the chest has been sufficiently released. Differences in compliance of the thoracic cage from one individual to another means that each individual will generally be able to support different amounts of force on the sternum without significant displacement occurring.
Increasingly, automated external defibrillators (AEDs) are used by rescuers treating victims of cardiac arrest for the delivery of defibrillatory shocks with the minimum of delay. The algorithms contained in the currently-available AEDs call for ‘hands off’ periods during which electrocardiographic (ECG) analysis is performed by the device and the rescuer withholds compressions. Compressions must be withheld because the accuracy of current rhythm analysis algorithms in AEDs is severely degraded by the artifact induced by the chest compressions. These AEDs also call for the rescuer to check for pulse or for signs of circulation during which time no compressions are performed. It has been shown in several studies that interruptions in the performance of chest compressions of as short a time as 20 seconds can dramatically reduce the probability of the return of spontaneous circulation (ROSC), a key survival measure. Other studies have also shown that the minimum amount of time required for the ‘hands off’ period is 20 seconds. There is therefore a need for the ability of AEDs to perform rhythm analysis while the rescuer continues with the chest compressions uninterrupted.